How do you form a massive black hole binary? (Part 1)

The science news media has been buzzing over the last fortnight after the LIGO (Laser Interferometer Gravitational-wave Observatory) team announced the discovery of gravitational waves. The signal appears to have come from the merger of two black holes, each approximately 30 times the mass of our Sun, that were orbiting each other and have since coalesced. Many posts have been devoted to this amazing discovery and its implications, but I'd like to talk about how the binary black hole system might have formed and what it means for our understanding of black holes, starting in this post with how you might form such massive black holes.

Artists impression of the collision of two black holes, as detected by the Laser Interferometric
Gravitational-wave Observatory (Credit: Vox.com)

First, a little background. Black holes broadly come in two varieties: stellar-mass black holes, with a mass a few times that of our Sun, and supermassive black holes, with millions of times the mass of our Sun. The less massive variety are found throughout galaxies, such as the X-ray binary Cygnus X-1, which was the first black hole discovered (see image below), while their more massive cousins are thought to lie at the centres of most large galaxies like our own.

Artist's impression of Cygnus X-1,
a black hole accreting material from its
companion star (Credit: Chandra X-ray Observatory)

How supermassive black holes form isn't really understood at the moment, but fortunately we do have a good idea how stellar mass black holes form. When very massive stars (O- and B-type stars) have exhausted all the nuclear fuel in their cores then they can no longer produce the necessary energy to support the weight of their heavy outer layers. Under the force of gravity the star collapses in on itself, a process known as core collapse. This collapse is incredibly violent and leads to two things, the first being the release of an immense amount of energy (a supernova) and the second is the creation of a very dense remnant such as a neutron star or a black hole.

This process is well understood and there is considerable evidence for it, as well as for the existence of black holes. However, the gravitational wave source that was observed in September wasn't just a black hole, it was two black holes orbiting each other in a binary system, and each black hole was thirty times the mass of our Sun, which is quite large for a typical black hole! So how do you make a black hole this big?

Wolf-Rayet star #124, as imaged by the Hubble
Space Telescope, showing all the material thrown
off by the star as it nears the end of its life
(Credit: HST)

The main factor determining the mass of a black hole is how much material the massive star that it formed from expelled during its life. This material might be expelled by stellar winds during the late stages of its life, such as while the star is a red supergiant or a Wolf-Rayet star (see image to the right), or much of the material might be expelled when the star explodes as a supernova.

The best way to prevent a star from losing most of its mass is to weaken its stellar winds so that they don't expel much material. The strength of a star's winds is usually proportional to the amount of elements like carbon, nitrogen or oxygen (known as metals to astronomers) that the star has. These elements accelerate the loss of material from stellar winds, so their absence would help to reduce the amount of mass lost and therefore increase the mass of the black hole produced.

Scientists have even gone as far as estimating that to produce a black hole as massive as those in the recent binary black hole merger they'd need to form from a massive star with as little as one hundredth the amount of metals that there are in our own Sun. Since the amount of these metals in a typical star has increased over the lifetime of the Universe, you'd need to form these stars very early in the Universe for them to have few enough metals to produce such massive black holes.

However, you could also make such massive black holes by other processes, possibly starting with a smaller black hole and slowly growing it. Material falling into black holes is absorbed by them, increasing their mass, though this is surprisingly hard to do efficiently, so it would be a slow process. Alternatively you could grow a black hole more rapidly by merging it with another black hole, in exactly the same way that the gravitational wave source that was detected was a merger of two black holes.

Whatever the process, the discovery of this merging black hole binary system suggests that black holes as massive as this must be relatively common in the Universe for scientists to have observed such a merging system so easily. Whether the black holes formed with such masses, or if they grew by accretion or mergers we may never know until we are able to detect and study more black holes and better understand the properties of these elusive but amazing objects.